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From: "Grein, Christoph" <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Re: Problems with 'class, help anyone?
Date: Mon, 11 Nov 2002 10:03:59 +0100 (MET)
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Date: 2002-11-11T10:03:59+01:00

> There is another way, with perhaps even less incompatibility: let's introduce
> third mode for a formal parameter - "return" mode (in addition to conventional
> "in" and "out"). That "return" may be applied to the procedure's first 
parameter
> only, and it means the same as "out" mode, with single exception: if the first
> parameter of a procedure is in "return" mode then the procedure *may* be 
called
> without the corresponding (first) argument, and in such a case it returns 
value.
> For example:
> 
> procedure Two_Fold(Result : return Boolean; Something : in Integer);
> 
> X : Boolean;
> Two_Fold(X, 0);  -- "return" as "out", procedure does not return value
> 
> if Two_Fold(0) then  -- first argument omitted, procedure returns value
>   ...
> end if;

But an out parameter and a return value are not the same, i.e. they cannot 
generally be interchanged!

Consider e.g.
     function F return String;
vs.
     procedure P (S: out String);
The former has no idea how long the object is that will get the result (if any), 
the 
latter takes the length from the parameter.